The state of the art includes WO 03/004915 (Stockstill), which describes a pipeline laying ship with hull and a deck area that supports pipe reels. A pipe joint storage area is provided for containing multiple joints of pipe. One or more pipeline welding stations are provided on the deck next to the reels, the pipeline welding stations being positioned to join the joints of pipe together to form an elongated pipeline that can be wound up on a selected reel. A tower is provided for guiding the pipeline as it is unwound from the reel, and the tower includes a bend controller, a straightener and a tensioner. The tensioner carries the weight of the pipeline between seabed and vessel. The tower can be positioned aft for launching the pipeline from the hull stern, and amidships for launching the pipeline through a vertical hull opening (so-called “moon pool”).
The state of the art also includes U.S. Pat. No. 5,346,333 (Maloberti, et al.), which describes a ship for laying flexible conduits on an ocean floor by continuous unrolling of the conduit at a laying site from at least one supply ship to the laying ship, wherein the flexible conduit is gradually transferred from the supply ship to a storage means located on the laying ship. The laying ship and the supply ship are equipped with means for storing flexible conduits. The laying ship comprises dynamic positioning means, a storage reel of flexible tubular conduit, and a tower positioned above a moon pool and having i.a. a radius controller and tensioners. The radius controller (guide means) comprises a chute making it possible for flexible tubular conduit to take a vertical path in the tower, towards the tensioners. The tensioners are placed below the chute, for example two tensioners are mounted on the tower of approximately parallelepipedic rectangular shape, in series. The tensioners are placed vertically “downstream” from the chute and “upstream” of a work table. The tensioners support the weight of flexible tubular conduit as it is suspended between the ship and the ocean floor. The tensioners comprise a plurality of tracks (caterpillar tracks), which exert a tightening force on the tubular conduit. The simultaneous advance of the tensioners tracks exert a tightening force on the tubular conduit, making possible the lowering of the tubular conduit to the seabed.
The state-of-the-art ships rely on the interaction between—and synchronization of—multiple tensioners in a tower in order to control the laying process. Operations at larger sea depths require more tensioner holding power, which in turn requires larger and taller towers on the laying vessels.
The present applicant has devised and embodied this invention to overcome the shortcomings of the prior art and to obtain further advantages.